When power is initially applied to an electric motor, and the motor begins to accelerate from rest, the electric motor must overcome several boundaries, one of which includes an initial inertia. Furthermore during the starting cycle or the time during which the motor accelerates up to its operating speed, also known as run up, the torque developed by the motor at any given instant needs to exceed the torque required by the load at that speed. If the torque developed by the motor fails to exceed the torque required by the load, the load will begin to decelerate and the motor may eventually stall.
At any given speed during run up, the difference between the motor torque and the load torque is known as the accelerating torque. A positive accelerating torque will result in the speed of the motor increasing to its operating speed. Taken over the complete curve of torque versus speed as the speed of the motor increases from rest to its operating speed, the accelerating torque in combination with the load moment of inertia determines the run up time.
During this time a large amount of current is required by the motor to overcome the torque imposed by the load. For power systems having limitations in the amount of power they are capable of supplying, for example a stand-by uninterruptable power supply, the large current requirement during run up may present a couple of problems. First, if the current is large enough it may trip an over current monitor, which may result in the system temporarily disabling itself for a predetermined period of time. Second, even if the current does not trip the over current monitor, the system may inherently compensate for the increased current by reducing the output voltage. This is because power is a function of voltage and current, wherein P=V .times.I. For systems at their power limit, in order for current to increase, voltage must correspondingly decrease. The corresponding decrease in voltage can cause some motors to stall or slip thereby preventing proper operation.
Thus, there continues to be a need for starter circuits which can supply appropriate starting currents to respective motors during run up. Preferably such functionality can be provided without unduly increasing circuit complexity or cost.